Validate input values and other conditions

Arguments

A list of tests. Each test should equal NULL for success,
FALSE for silent failure, or a string for failure with an error
message.

errorClass

A CSS class to apply. The actual CSS string will have
shiny-output-error- prepended to this value.

expr

An expression to test. The condition will pass if the expression
meets the conditions spelled out in Details.

message

A message to convey to the user if the validation condition is
not met. If no message is provided, one will be created using label.
To fail with no message, use FALSE for the message.

label

A human-readable name for the field that may be missing. This
parameter is not needed if message is provided, but must be provided
otherwise.

Description

For an output rendering function (e.g. renderPlot()), you may
need to check that certain input values are available and valid before you
can render the output. validate gives you a convenient mechanism for
doing so.

Details

The validate function takes any number of (unnamed) arguments, each of
which represents a condition to test. If any of the conditions represent
failure, then a special type of error is signaled which stops execution. If
this error is not handled by application-specific code, it is displayed to
the user by Shiny.

An easy way to provide arguments to validate is to use the need
function, which takes an expression and a string; if the expression is
considered a failure, then the string will be used as the error message. The
need function considers its expression to be a failure if it is any of
the following:

If any of these values happen to be valid, you can explicitly turn them to
logical values. For example, if you allow NA but not NULL, you
can use the condition !is.null(input$foo), because !is.null(NA)
== TRUE.

If you need validation logic that differs significantly from need, you
can create other validation test functions. A passing test should return
NULL. A failing test should return an error message as a
single-element character vector, or if the failure should happen silently,
FALSE.

Because validation failure is signaled as an error, you can use
validate in reactive expressions, and validation failures will
automatically propagate to outputs that use the reactive expression. In
other words, if reactive expression a needs input$x, and two
outputs use a (and thus depend indirectly on input$x), it's
not necessary for the outputs to validate input$x explicitly, as long
as a does validate it.